A fixed interrogation unit 10 as shown in FIG. 1, is capable of communicating or exchanging messages with a plurality of electronic cards 11-16 that are mobile with respect to the interrogation unit. The communication is wireless, i.e., the messages are transmitted by RF or microwave electromagnetic carrier waves. The transmission channel is thus formed by the ambient atmosphere.
An electronic card 11 may be an electronic module, a badge or a chip card, and may be carried by an individual, a vehicle, an instrument, cattle, etc. The interrogation unit 10 may be contained in a base station, an access control terminal, an on-the-fly toll-gate, etc. As an alternative, the interrogation unit 10 is mobile and the electronic cards 11-16 are fixed. As another alternative, the interrogation unit 10 and the electronic cards 11-16 are both mobile.
In all three cases, the electronic cards 11-15 may be located within a zone 20 centered around the interrogation unit 10. The zone 20 is the range of the messages exchanged, whereas other electronic cards 16 may be outside this zone. In FIG. 1, the boundary of this zone 20 is indicated by a dashed line 25. The boundary of a zone 20 may vary in time, for example, because of the presence of foreign objects forming a shield against the propagation of the electromagnetic waves.
The volume of air contained in the zone 20 forms the channel for the transmission of the messages exchanged between the interrogation unit 10 and the electronic cards 11-15. This channel is unique and has to be time-shared according to a communications protocol between the interrogation unit 10 and the electronic cards 11-15. This protocol is of the master/slave type. Each exchange of a message between the interrogation unit 10 (master) and a specified electronic card 11 (slave) is initiated by the interrogation unit.
By default, the electronic cards 11-15 are in a state such that they cannot receive control messages sent by the interrogation unit 10 and, therefore, are even less capable of answering these messages. In other words, the electronic cards 11-15 are referred to as being in an asleep state. This is why, before sending a control message intended for a specified electronic card 11, the interrogation unit 10 must first of all awaken each electronic card 11 to place it in a state enabling it to receive the control message and, as the case may be, to respond to it by sending a response message. The interrogation message contains a particular message W, called an "awakening" message, intended for a particular electronic card 11. After sending the control message and, if necessary, after reception of the response message, the interrogation unit 10 sends a particular message S called a "putting-to-sleep message" for the electronic card 11. The electronic card 11 then goes back into the state in which it cannot receive any control messages.
The awakening message W and the putting-to-sleep message S each have a unique parameter. This parameter is a number for the identification of the electronic card 11 for which these messages are intended. This is why they will respectively be written as W(X) and S(X), where X is the value of an identification number enabling the identification of the electronic card 11 for which the control message is intended. An identification number of this kind is uniquely assigned to each electronic card 11. In other words, a single identification number is associated with each electronic card 11.
However, because of the mobility of the electronic cards 11-15 and/or its own mobility, the interrogation unit 10 has no a priori knowledge of whether the electronic cards 11-15 are in the zone 20 corresponding to the range of the messages. As the case may be, the interrogation unit 10 does not know the number of electronic cards 11-15 present and does not know which of the electronic cards are present.
This is why implementation of an identification system requires a method by which the interrogation unit 10 can identify the electronic cards 11-15 present within the zone 20, and corresponds to the range of the messages to be exchanged.
Hereinafter, this zone 20 is called the "investigation zone" inasmuch as it is the zone within which such a method detects and identifies the electronic cards 11-15 present. To identify an electronic card that is present is to identify its identification number. It is the knowledge of the identification numbers of the electronic cards 11-15 present that provides for the management of the exchanges of the control messages.
In the prior art, identification methods of this kind have already been proposed. These known methods implement an arborescent iterative algorithm enabling the bit-by-bit reconstruction of the identification number of each electronic card present in the investigation zone 20. To put it briefly, it is possible to represent all the values of identification numbers in a tree-like structure.
According to the terminology proper to this type of structure, as used especially in the field of computers, the root of the tree provides two branches. Each branch ends in a node corresponding to the logic value 1 and 0, respectively, of a first bit of these identification numbers, for example, the most significant bit. These two nodes each give rise to two new branches that each end in a node corresponding to the logic value 1 and 0, respectively, of another bit of the identification number, for example, the immediately less significant bit. Thus, the method continues in the same way up to the last generation of the tree starting from the root. The tree has as many generations as there are bits to encode with respect to the identification numbers.
In other words, each node other than those of the first generation has a parent corresponding to the logic value of the more significant bit. The first generation corresponds to the logic value of the most significant bit. Each node other than the node of the last generation has two children corresponding to the logic value of the less significant bit. The last generation corresponds to the value of the least significant bit of the identification number. Any journey made in the tree from the root to the node of the last generation enables a logic value 1 or 0 to be assigned to each bit of the identification number, beginning with the most significant bits in the example.
The principle of the known methods lies precisely in travelling through the tree of FIG. 2 to deduce therefrom the identification numbers of the electronic cards 11-15 present in the investigation zone 20. More specifically, these methods include the steps of sending, by the interrogation unit 10, of interrogation messages intended for the electronic cards 11-15 present in the investigation zone 20, or at least for a group of such cards which are authorized to respond, and sending response messages by the cards.
The interpretation of the response messages received by the interrogation unit 10 at each iteration of the algorithm makes it possible to go forward in the tree of the identification numbers by identifying the value of an additional bit of the identification number of at least one of the electronic cards 11-15. In the known method of identification, each response message sent by an electronic card 11 authorized to respond includes the full identification number of the electronic card.
When several electronic cards 11-15 are simultaneously in the investigation zone 20 and are authorized to respond, several electronic cards may simultaneously send a response message. There is then a collision for the electronic signals that transit simultaneously on the transmission channel. These collisions get added up and, ultimately, corrupt one another. In particular, there may be a collision between the bits of the identification numbers sent by different electronic cards 11-15 responding simultaneously to an interrogation message sent by the interrogation unit 10.
At present, there are known methods that make it possible to overcome this problem of collision, or even use this phenomenon according to a particular algorithm of identification. The tree-like iterative methods known in the prior art lend themselves well to implementation by a software method which, as is known, is particularly simple to perform with currently used, reliable and economical electronic circuits 11-15, such as microcontrollers. However, these known methods have, among other drawbacks, the specific feature of being too slow inasmuch as the exploration of the entire tree of the identification numbers is lengthy, and all the more lengthier as the identification number is encoded on a large number of bits.
In certain applications, it is necessary to provide for the coexistence of a large number of different electronic cards 11-15 to provide for a larger number of bits to encode their respective identification numbers. At the same time it must be known that, in most cases, only a very small number of them will be simultaneously present in the investigation zone 20. This is the case, for example, in an on-the-fly toll-gate system where the electronic cards 11-15 are transponder-based modules fitted into each automobile whose owner is a subscriber to the service concerned, or where the interrogation unit 10 is contained in a fixed toll-gate terminal.